Troubleshooting help

I took the new (to me) 7.5” 300BLK out today for the first time. Had five different ammo types (2 super, 3 sub) loaded up in five 5.56 PMags. Never had any issues with any of those ammo choices, or PMags, in my 10” SBR.

Had multiple occurrences of the next round not loading, but empty brass extracted and ejected and trigger reset. Bolt didn’t lock back on empty mag multiple times. This happened suppressed and unsuppressed.

I know we had the 300BLK in 5.56 PMags discussion this week in another thread, but the not locking back on an empty mag makes me think it’s a gas issue. Previous owner said he bought it new and probably put <50 rounds through it.

I didn’t try any troubleshooting at the range. Could’ve swapped it to a known good lower, swapped BCGs, etc...but decided to leave it be and shoot the other firearms I’d brought with me.

Plan is to check gas block alignment tomorrow if possible. Anything else I can check without being at the range?

Not enough gas was my first thought.......especially if its not throwing the brass out with some authority.

With a fairly new rifle.....be sure the bolt is clean and lubed.

BowWow said:

Not enough gas was my first thought.......especially if its not throwing the brass out with some authority.

With a fairly new rifle.....be sure the bolt is clean and lubed.

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Where the brass went was one thing I should’ve paid attention to.

I’ve had the same thing happen with a misaligned gas block years ago. It sounds like it’s undergassed but just barely.

If not the gas block, I would try a different BCG and a lighter buffer (if using a heavy one).

Although I’ve built a dozen or more lowers, I’ve never built an upper, just tinkered with them a bit. I thought the gas block needed to be installed with a small gap between it and the shoulder (or whatever it’s called at that point) on the barrel. Looked at it this morning and it appeared to be pushed up again the shoulder.



So I loosened it up and removed it just to check things out. Gas tube was clear. Barrel has no dimples for gas block. However, the ring left on the barrel from the gas block hole seemed to go around the gas port, but definitely not centered. I have an adjustable gas block on my 10” SBR and it’s turned almost completely off and everything functions fine. So even if the holes weren’t perfectly lined up, I think it still was getting enough.

I lubed the BCG up real good (it may have been a little dry) and will just try it again soon before I start tinkering. But I’ll take another BCG with me as the first thing to try. I might have another buffer here, but it might be the same as what’s in it...since most of my builds have been PSA build kits. But I can weigh any of the ones I find and see if there’s a difference.

I did check with the previous owner. He had a similar issue with two rounds, but at the time he’d attributed it to subs somebody else had loaded and the Law folding adapter he’d installed. That leads me to think it isn’t my mags.

Not sure if this means anything, but it’s bouncing brass all over the deflector.



Keep in mind that’s less than 100 rounds total.

Carry a couple of different mags and good luck!

I would definitely try a few springs and lighter buffers as it does indeed appear to be undergassed.

I had to open up the gas port a few thousands on my 300BLK SBR for subs.
Find what a common diameter is and see what sized drill bit (smooth end) fits.

Pink_Vapor said:

I had to open up the gas port a few thousands on my 300BLK SBR for subs.
Find what a common diameter is and see what sized drill bit (smooth end) fits.

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What is the gas port diameter spec? Im thinking its .062 but maybe larger for the 300BO?

BowWow said:

What is the gas port diameter spec? Im thinking its .062 but maybe larger for the 300BO?

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I don’t recall, it was years ago when I did it. I would start by going to several reputable barrel dealers and look up their specs.

I had the same issue with my 300blk pistol. I built it from scratch and used an ar15 buffer spring.

My buddy gave me a lighter buffer spring that he said was made for .300blk ARs.

I put it in and never had another malfunction.

Alabamacoastie said:

I had the same issue with my 300blk pistol. I built it from scratch and used an ar15 buffer spring.

My buddy gave me a lighter buffer spring that he said was made for .300blk ARs.

I put it in and never had another malfunction.

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I may look for that. Just found it odd my 10” never gave me issues.

Jerzsubbie said:

If not the gas block, I would try a different BCG and a lighter buffer (if using a heavy one).

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NKD said:

I would definitely try a few springs and lighter buffers as it does indeed appear to be undergassed.

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Free fixes are always nice. The buffer that’s in there came from a Spike’s pistol buffer tube kit. It weighs 4.1oz. I checked my box and found a 2.9oz version that’s the same length.

So, I’ll try it again before changing anything. Want to confirm which ammo(s) are having the issue. But I’ll have a lighter buffer with me as something to test.

BigWaylon said:

Free fixes are always nice. The buffer that’s in there came from a Spike’s pistol buffer tube kit. It weighs 4.1oz. I checked my box and found a 2.9oz version that’s the same length.

So, I’ll try it again before changing anything. Want to confirm which ammo(s) are having the issue. But I’ll have a lighter buffer with me as something to test.

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4.1oz would be an ST-T2, slightly lighter than an H2 (4.6oz), but much heavier than a standard Carbine buffer which is 3oz. If it’s just the buffer and/or spring, once you get the correct weight buffer in there, you’ll have a very soft shooting gun since it won’t be over gassed!

Jerzsubbie said:

4.1oz would be an ST-T2, slightly lighter than an H2 (4.6oz), but much heavier than a standard Carbine buffer which is 3oz. If it’s just the buffer and/or spring, once you get the correct weight buffer in there, you’ll have a very soft shooting gun since it won’t be over gassed!

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Yes, it’s marked ST-T2.

I thought everybody agreed it was under gassed, not over gassed?

BigWaylon said:

Free fixes are always nice. The buffer that’s in there came from a Spike’s pistol buffer tube kit. It weighs 4.1oz. I checked my box and found a 2.9oz version that’s the same length.

So, I’ll try it again before changing anything. Want to confirm which ammo(s) are having the issue. But I’ll have a lighter buffer with me as something to test.

Click to expand...

I bet that fixes it right up. Maybe the 4.1 oz is just a bit heavy.

BigWaylon said:

Yes, it’s marked ST-T2.

I thought everybody agreed it was under gassed, not over gassed?

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Correct, it is under-gassed given the current configuration. ARs are often over gassed for reliability and variability of ammo. Assuming a full auto carrier, that gas has to move 11.5oz BCG + 4.1oz buffer + force of the spring and it can’t do it. You need to either increase the force (gas) or decrease the mass (buffer/BCG/spring). If shaving off 1oz is all it takes, it’ll be gassed perfectly. Trying a lighter spring may help too.

I went through the details of tuning the reciprocating mass of my competition rifle quite extensively, partially to nerd out, but also because I wanted the softest shooting .223. Using an adjustable gas block, lightweight spring, lightweight carrier, lightweight buffer, and great brake, now my rifle shoots like a dream.

Jerzsubbie said:

Correct, it is under-gassed given the current configuration. ARs are often over gassed for reliability and variability of ammo. Assuming a full auto carrier, that gas has to move 11.5oz BCG + 4.1oz buffer + force of the spring and it can’t do it. You need to either increase the force (gas) or decrease the mass (buffer/BCG/spring). If shaving off 1oz is all it takes, it’ll be gassed perfectly. Trying a lighter spring may help too.

I went through the details of tuning the reciprocating mass of my competition rifle quite extensively, partially to nerd out, but also because I wanted the softest shooting .223. Using an adjustable gas block, lightweight spring, lightweight carrier, lightweight buffer, and great brake, now my rifle shoots like a dream.

Click to expand...

I love the low mass recoil feel. All my rifles are now adjustable gas. Using a -10% sprinco spring, 1/2 oz plastic buffer, and LW BCG.

The brake does 90% of the work, but on the closing end of the recoil cycle the low mass stuff really smooths it out.